The invention relates to the use of chiral and/or achiral compounds with a thiazole or thiadiazole structural element as components in media having a smectic liquid-crystalline phase. The invention also relates to media having a smectic liquid-crystalline phase, in particular having a chiral tilted smectic liquid-crystalline phase, which media contain compounds having a thiazole or thiadiazole structural element.
Media having chiral tilted smectic liquid-crystalline phases and ferro-electric properties can be prepared by adding a suitable chiral doping agent to base mixtures having one or more tilted smectic phases (L. A. Beresnev et al., Mol. Cryst. Liq. Cryst. 89, 327 (1982); H. R. Brand et al., J. Physique 44 (lett.), L-771 (1983)). Such media can be used as dielectrics for rapidly switching displays, which are based on the principle, described by Clark and Lagerwall, of SSFLC technology (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (t980); U.S. Pat. No. 4,367,924) as a result of the ferro-electric properties of the chiral tilted phase.
In this phase, the elongate molecules are arranged in layers, the molecules having a tilt angle relative to the perpendicular of the layer. When passing from layer to layer, the direction of tilt changes by a small angle relative to an axis perpendicular to the layers, so that a helix structure is formed. In displays based on the principle of SSFLC technology the smectic layers are arranged perpendicular to the plates of the cell. The helix-like arrangement of the tilt direction of the molecules is suppressed by a very small spacing of the plates (about 1-2 .mu.m). As a result, the longitudinal axes of the molecules are forced to arrange in a plane parallel to the plates of the cell causing two preferential tilt orientations. Switching between these two states in the liquid-crystalline phase showing spontaneous polarization is possible by applying a suitable electric alternating field. This switching process is substantially faster than in conventional twisted cells (TN-LCD's) based on nematic liquid crystals.
In many applications, a great disadvantage of the currently available materials having chiral tilted smectic phases (such as, for example, Sc*, but also S.sub.H *, S.sub.I *, S.sub.J *, S.sub.K *, S.sub.G *, S.sub.F *) is their low chemical, thermal and light stability. A further disadvantageous property of displays based on currently available chiral tilted smectic mixtures is that the spontaneous polarization has unduly small values and/or the viscosity is unduly high, so that the switching time behavior of the displays is unfavorably affected and/or the pitch and tilt of the phases do not meet the requirements of display technology. Furthermore, the temperature range of the ferro-electric phases is in most cases too small and predominantly is at unduly high temperatures.